Field of the Invention
The present invention relates to a device for expanding a laser beam, in particular a UV laser beam, and an associated method for expanding a laser beam.
Background of the Invention
UV lasers, that is to say, lasers which produce laser radiation in the UV wavelength range (with wavelengths below the visible wavelength range) are becoming increasingly significant, particularly in microprocessing operations. This increases the demand for UV lasers of high and medium power. The production of ultraviolet laser radiation is often carried out by means of frequency conversion of an infrared laser beam in non-linear crystals. This frequency conversion requires on the one hand high infrared intensities and small beam divergence in the non-linear crystals in order to enable efficient conversion from the infrared wavelength range to the ultraviolet wavelength range. On the other hand, the intensity, in particular that of the UV radiation produced, may not become too high, in order to prevent the crystal from being destroyed or a rapid degradation of the optical properties thereof.
These requirements, for example (but not exclusively) in the case of ultra-short pulse lasers with UV pulse lengths in the range of approximately 5 ps and UV pulse energies of more than 1 μJ or in the case of pulse lengths of approximately 1 ps and UV pulse energies of more than 0.2 μJ, may lead to collimated or almost collimated UV laser beams of high intensity, whose diameter does not increase significantly over the extent of the conversion structure, that is to say, whose intensity does not significantly reduce over this length.
Permanent UV radiation with high intensity leads in transmissive optical units (for example, lenses or other optical units of quartz glass) and optionally also in the anti-reflective coatings thereof to undesirable permanent material changes which can impair the transmission properties of these transmissive optical units to the point of being unusable. Traces of various gases (for example, hydrocarbon or silicon compounds) in an imprecisely controlled environment of the optical units can also be changed by means of the UV radiation in such a manner that deposits occur on the optical units and may also lead to them becoming unusable after a period of time. In particular with very high intensities of the UV radiation, this degradation can occur so rapidly that economic use of transmissive optical units for UV laser beams is no longer possible.
In order to make collimated UV laser beams of high intensity produced in the manner described above usable for applications, it is necessary to expand them in such a manner that the intensity thereof over the desired period of use does not lead to damage of the following optical units (for example, beam guiding, beam shaping or processing optical units).
Although such an expansion could be achieved by means of a large propagation length of the laser beam as far as the following optical units, so that the laser beam would already per se become sufficiently large, in the present case, in which the laser beam is (almost) collimated, this is difficult to implement since a very high propagation length would be required and the structural size of the entire expanding optical unit is limited.
Conventionally, for beam expansion of (almost collimated) laser beams, devices are therefore used for beam expansion, for example, in the form of a Galilei objective lens 1 (cf. FIG. 1). Such an objective lens 1 has at least two lenses 2, 3 in order to convert an input collimated laser beam 4 into an (in this case 6 times) expanded similarly collimated laser beam 5. The Galilei objective lens 1 constitutes a beam expansion optical unit of simple construction and can be configured, with short structural length and typically small structural space using axially arranged spherical lenses 2, 3, in such a manner that they lead to very small beam deformations with a high level of tolerance with respect to the input beam position (that is to say, tilting or displacement of the input beam). With regard to the output beam divergence, radius errors of the lenses 2, 3 or errors in the input beam divergence can be compensated for by means of simple axial displacement of one of the lenses 2, 3, without a lateral displacement of the expanded laser beam 5 occurring.
However, in a disadvantageous manner the input-side lens 2 of the Galilei objective lens 1 is itself subjected to the high UV intensity of the laser beam 4 which has not yet expanded so that, in particular when a dielectrically anti-reflection-coated input-side lens 2 is used, an unacceptable degradation of the transmission properties thereof may occur. As a partial solution for this problem, the Galilei objective lens 1 may be operated with an uncoated input optical unit, that is to say, without any anti-reflective coating. However, power losses are produced in this instance owing to Fresnel reflexes on the uncoated lens 2. The problem of the radiation-related change or degradation of the lens material also remains.
It has been found that the impairment of the reflection properties of dielectric mirrors with intensive UV radiation is substantially smaller than the impairment of the transmission properties of transmissive optical units and in particular dielectrically anti-reflection-coated transmissive optical units. Therefore, mirror objective lenses for the beam expansion of UV radiation may achieve significantly longer operating times than lens systems.
FIG. 2 shows a telescope arrangement 6 having two spherical folding mirrors 7, 8 as reflective optical elements in a Z-shaped folding arrangement (in this case 6 times) for converting an input collimated laser beam 4 into an expanded collimated laser beam 5. The spacing between the folding mirrors 7, 8 is in this instance approximately 150 mm, the folding angle 2α which is identical for both folding mirrors 7, 8 is approximately 20°.
However, a non-axial mirror system for beam expansion, as shown, for example, in FIG. 2, generally has various disadvantages. Either aspherical optical elements are required in order to reduce astigmatism and coma which makes these systems expensive and sensitive to adjustment, or such systems, with a given tolerance of the application with respect to imaging errors, are limited to relatively large mirror radii and small folding angles, whereby a large structural form of the entire device is necessary, and/or these systems are limited to quite specific radius and folding angle combinations and beam properties (for example, convergent output beam (intermediate focal point)).
For the arrangement shown in FIG. 2, FIG. 3 shows four spot diagrams of the remote field of the mirror telescope 6 produced by means of simulations (“ray tracing”). For the input laser beam 4, a Gaussian intensity distribution was assumed, in which the 1/e2 diameter (that is to say, the diameter in which the intensity has dropped to 1/e2 times the maximum value) was 0.83 mm, that is to say, the output laser beam 5 should ideally have a 1/e2 diameter of 5 mm with the 6-times enlargement shown here. The pupil of the simulated mirror-telescope 6 was selected in this instance in such a manner that the calculated radius (of approximately 42 μrad) of the Airy disc 9 for a UV wavelength of 343 nm approximately corresponds to the divergence angle of a laser beam with Gaussian intensity distribution and 1/e2 diameter of 5 mm.
In addition to an axially orientated input beam, which is illustrated at the top left-hand side in FIG. 3, three additional beams with 0.2° deviation from the input axis of the laser beam were calculated and are illustrated at the top right and at the bottom left and bottom right. The scale size S1 of the (square) angular range shown in FIG. 3 is in this instance 2000 μrad. Since the spots shown in FIG. 3 are substantially outside the circular Airy disc 9, the mirror telescope 6 is far from being limited in terms of diffraction for the input laser beam 4.
In general, with non-axial mirror telescopes such as the mirror telescope 6 shown in FIG. 2, the radius tolerances of the folding mirrors 7, 8 or defects in the collimation of the input beam 4 cannot be compensated for by simple displacement of an individual folding mirror 7, 8 without an undesirable lateral displacement of the output laser beam 5 occurring.
DE 10 2007 009 318 A1 also relates to the problem of the expansion of a high-energy laser beam. The solution described therein makes provision for the laser beam to be expanded by means of a transmitting or reflecting optical element in such a manner that the energy density which is to be associated remains below a critical energy density in order to prevent irreversible damage to the subsequent optical components. For beam expansion, in an embodiment, a cylindrical convex mirror which can be inclined through 45° relative to the laser beam and a convex cylinder lens are used. Owing to the inclination of the convex mirror through 45°, the energy density on the mirror surface is intended to be reduced by a factor of root 2 so that the mirror itself does not become damaged. The cylinder lens is intended to move the divergent expanded laser beam into a parallel beam path with an increased beam cross-section. However, this affords only the possibility of expanding the laser beam in one direction.
As an alternative to expansion in two directions, in another embodiment of DE 10 2007 009 318 A1, it is proposed to convert a device for beam expansion 10 (cf. FIG. 4) having a spherical-convex mirror 7 for beam expansion and two cylinder lenses 11, 12 which are orientated with their cylinder axes orthogonal relative to each other for converting the laser beam which is expanded in a divergent manner on the convex concave mirror 7 into an output collimated laser beam 5, that is to say, with a parallel beam path. Owing to the use of the cylinder lenses 11, 12, however, the device 10 is very susceptible to adjustment, as illustrated below with reference to FIG. 5 and FIG. 6.
FIG. 5 is an illustration similar to FIG. 3, the properties of the input laser beam 4 and the pupil corresponding to that of FIG. 3, but the scale size S2 being smaller and being 200 μrad. In the spot illustration of the far-field illustrated at the top right-hand side in FIG. 3, all the beams or spots are located within the Airy disc 9, that is to say, the root-mean-square (RMS) radius of the beam distribution is significantly smaller than the radius (approx. 42 μrad) of the Airy disc 9, so that the optical unit shown in FIG. 4 is limited in terms of diffraction for the expanded laser beam 5 as long as the input beam is axially orientated. However, with an oblique beam incidence, small impairments of the imaging properties are produced, as can be seen with reference to the spot illustrations in FIG. 5 at the top left-hand side, bottom left-hand side and bottom right-hand side, in which the field angle of the incident laser beam 4 has been changed in various directions by 0.2°, the inlet aperture being located 150 mm in front of the convex mirror 7.
FIG. 6 is an illustration of a far-field spot diagram similar to FIG. 3 and FIG. 5, in which both the properties of the incident laser beam 4 and the pupil (and the scale size S2) have been selected as at the top left in FIG. 5, but one of the cylinder lenses 11, 12 was tilted in each case through 0.2° about the beam direction. As can be seen clearly, a small misalignment of a respective cylinder lens 11, 12 leads to a powerful astigmatism of below approximately 45°, which leads to the far-field distribution not being limited in terms of diffraction since a plurality of spots come to rest outside the Airy disc 9 (radius approximately 37 μrad).